1. Field of the Invention
This invention relates to an optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein the recording layer is irradiated with a laser beam to change its crystalline state whereupon a change in reflectivity by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
When information is recorded on the optical recording medium of phase change type, the entire recording layer is first brought into crystalline state, and then, a laser beam of high power (recording power) is applied so that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallization temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
The recording layer of phase change type is most often constituted from chalcogenide systems such as Gexe2x80x94Sbxe2x80x94Te system and Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system which provide a substantial difference in reflectivity between crystalline and amorphous states and have a relatively stable amorphous state.
The materials of Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system have the merit that signal output is high compared to the materials of Gexe2x80x94Sbxe2x80x94Te system. Accordingly, the materials of Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system are highly promising for use as a material in the high density recording wherein the recording at exceedingly higher density is sought.
In the experiments of the inventors of the present invention, however, the materials of Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system were found to exhibit higher susceptibility to crystallization of record marks when stored in high temperature environment compared to the materials of Gexe2x80x94Sbxe2x80x94Te system, namely, to suffer from poor high temperature reliability. It has been also found that the high temperature reliability of the materials of Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system further decreases with the increase in the number of overwriting operations.
In view of such situation, an object of the present invention is to improve the high temperature reliability, and in particular, the high temperature reliability after repeated overwriting operations of the optical recording medium wherein materials of Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system is used for the recording layer.
Such objects are attained by the present invention as described in (1) to (4), below.
(1) An optical recording medium having a recording layer of phase change type, wherein said recording layer contains Ag, In, Sb, and Te as its main components, and Ge as its sub-component such that, when molar ratio of the elements constituting the main and sub-components is represented by formula (I):
(AgaInbSbcTed)(1-e/100)Geexe2x80x83xe2x80x83(I)
2xe2x89xa6axe2x89xa620,
2xe2x89xa6bxe2x89xa620,
35xe2x89xa6cxe2x89xa680,
8xe2x89xa6dxe2x89xa640,
a+b+c+d=100, and
1xe2x89xa6exe2x89xa615.
(2) The optical recording medium according to the above (1) wherein exe2x89xa71.8.
(3) The optical recording medium according to the above (1) or (2) wherein exe2x89xa68.
(4) The optical recording medium according to any one of above (1) to (3) wherein 58xe2x89xa6cxe2x89xa680.